The purpose of a gas separator in a fuel cell assembly is to keep the oxygen containing gas supplied to the cathode side of one fuel cell separate from the fuel gas supplied to the anode side of an adjacent fuel cell, and to conduct heat generated in the fuel cells away from the fuel cells. The gas separator may also conduct electricity generated in the fuel cells between or away from the fuel cells. Although it has been proposed that this function may alternatively be performed by a separate member between each fuel cell and the gas separator, much development work has been carried out on electrically conductive gas separators.
Sophisticated ceramics for use in gas separators for solid oxide fuel cells have been developed which are electrically conductive, but these suffer from a relatively high fragility, low thermal conductivity and high cost. Special metallic alloys have also been developed, but it has proved difficult to avoid the various materials of the fuel cell assembly and the interfaces between them degrading or changing substantially through the life of the fuel cell, particularly insofar as their electrical conductivity is concerned, because of the tendency of different materials to chemically interact at the high temperatures that are required for efficient operation of a solid oxide fuel cell. For example, most metallic gas separators contain substantial quantities of the element chromium, which is used to impart oxidation resistance to the metal as well as other properties.
It has been found that where chromium is present in more than minute quantities it may combine with oxygen or oxygen plus moisture to form highly volatile oxide or oxyhydroxide gases under conditions that are typical of those experienced in operating solid oxide fuel cells. These volatile gases are attracted to the cathode-electrolyte interface where they may react to form compounds that are deleterious to the efficiency of the fuel cell. If these chromium reactions are not eliminated or substantially inhibited, the performance of the fuel cell deteriorates with time to the point where the fuel cell is no longer effective.
Several of these metallic alloys and one proposal for alleviating this problem are described in our patent application WO96/28855, in which a chromium-containing gas separator is provided with an oxide surface layer that reacts with the chromium to form a spinel layer between the substrate and the oxide surface layer and thereby tie in the chromium. However, these specialist alloys remain expensive for substantial use in fuel cell assemblies and it would be preferable to have a lower cost alternative.
Special stainless steels have also been developed that are stable at high temperature in the atmospheres concerned, but they generally contain substantial amounts of chromium to provide the desired oxidation resistance, and special coatings or treatments are required to prevent the chromium-based gases escaping from a gas separator formed of these steels. Another approach to a heat resistant steel gas separator is described in our patent application WO 99/25890. However, all of these heat resistant steels are specialist materials and their cost will remain high unless substantial amounts can be produced. Furthermore, the thermal and electrical conductivities of heat resistant steels are low relative to many other metals and alloys, for example 22-24 W/m.K compared to 40-50 W/m.K for the Siemens-Plansee alloy described in WO96/28855. To compensate for this, the thickness of the steel gas separator has to be increased, increasing the mass and cost of a fuel cell stack.
In yet another proposal, disclosed in our patent application WO 00/76015, we have found that copper-based gas separators may be successfully utilised in solid oxide fuel cell assemblies without poisoning the anode. Such a gas separator member comprises a layer of copper or copper-based alloy having a layer of oxidation-resistant material on the cathode side.
One of the major difficulties with developing a satisfactory gas separator is ensuring that its coefficient of thermal expansion (“CTE”) is at least substantially matched to that of the other components of the fuel cell assembly. For example, solid oxide fuel cells comprising an oxide electrolyte with a cathode and an anode on opposed surfaces operate at temperatures in excess of about 700° C., and the alternating gas separators and fuel cells are generally bonded or otherwise sealed to each other. Thus, any substantial mismatch in the CTE between the two components can lead to cracking of one or both of them, with resultant leakage of the fuel gas and oxygen-containing gas across the component or components, and eventually to failure of the fuel cell stack.
A particular difficulty with developing a suitable fuel cell gas separator is providing a material that provides all four functions of separating the fuel gas on one side from the oxygen-containing gas on the other side, being thermally conductive, having a CTE substantially matched to that of the other fuel cell components, and being electrically conductive.
In order to meet these requirements, it has been proposed to provide a gas separator formed principally of a material that may not be electrically conductive, or not adequately electrically conductive, but that meets the other requirements, and to provide electrically conductive feedthroughs through the thickness of the separator. One such proposal is made in Kendall et al. in Solid Oxide Fuel Cells IV, 1995, pp. 229-235, in which the gas separator plate is formed of a zirconia material and lanthanum chromite rivets extend through holes in the plate. Another proposal for electrically conductive feedthroughs through the thickness of the separator is made in EP 0993059. In this proposal, a ceramic gas separator plate, preferably stabilized zirconia, has passages therethrough that in the preferred embodiment are filled with cathode material from the cathode side and with anode material from the anode side. Alternatively, they may be filled with a single material composition such as doped chromite, silver-palladium or Plansee alloy.
Thus, the feedthrough material is different to that of the principal separator material and will generally have a higher electrical conductivity. However, as the gas separator is subjected to thermal cycling, this can lead to the disadvantage of the feedthrough material becoming loose in the plate material, due to their different CTEs, and to the leakage of gas through the passages in which the feedthroughs are formed.
Additionally in EP 0993059, individual contacts for the feedthroughs of, for example, Ni, Plansee metal or Ag—Pd on the anode side and Ag—Pd or lanthanum strontium manganite on the cathode side, are bonded to the respective electrode by means of a bond layer that overlies the entire electrode surface. Such a bond layer will tend to inhibit free gas flow through the electrode and the individual contacts must be located very accurately on the electrodes to overlie the respective feedthrough when the fuel cell plates carrying the electrodes and the individual contacts are assembled with the gas separator plates
An alternative proposal published in US Patent Application 20020068677 on 6 Jun. 2002 includes a gas separator plate in which the principal plate material is a high silica glass matrix having a metal conductor incorporated therein formed of a material such as silver, Ag—Pd alloy, gold and ferritic stainless steel.
An aim of each aspect the present invention is to provide a fuel cell gas separator that alleviates at least some of the abovementioned disadvantages.